1. Field of the Invention
The present invention relates to an optical deflector of an air bearing type used in an optical writing device in a laser printer, a digital copying machine, etc. and deflecting a modulated optical signal.
2. Description of the Related Art
An optical deflector of a rotary mirror type using a polygon mirror is widely used in an optical writing device in a laser printer, a digital copying machine, etc. since a large deflection angle is obtained in the optical deflector.
In the optical deflector of this type, a deflection speed depends on the number of rotations of the rotary polygon mirror. Accordingly, to obtain high resolution, it is necessary to rotate the polygon mirror at a high rotational speed such as several ten thousand rpm, e.g., 30,000 rpm. A bearing must have small friction to realize such a high rotational speed so that an air type bearing is generally used.
Japanese Patent Publication (KOKOKU) No. 53-6854 discloses an optical deflector using a rotary mirror and provided with a radial dynamic pressure bearing and a repulsive magnetic thrust bearing. The dynamic air radial bearing pivotally supports a rotary shaft having a polygon mirror fixed to an upper portion thereof by a dynamic air pressure in a radial direction of the rotary shaft. The repulsive magnetic thrust bearing is disposed in a lower portion of the rotary shaft and floats the rotary shaft in an axial direction thereof by magnetic repulsive force of a permanent magnet, thereby pivotally supporting the rotary shaft.
In this optical deflector using the rotary mirror, there is a problem that the rigidity of the repulsive magnetic thrust bearing is weak since this bearing is constructed by a bearing of a simple repulsive type disposed in the lower portion of the rotary shaft. Further, it is necessary to dispose dedicated parts such as a holder, damper parts, etc. to construct the repulsive magnetic thrust bearing. Therefore, the number of parts is increased and the cost of the optical deflector is increased. Further, a space for the repulsive magnetic thrust bearing is increased so that the entire optical deflector is large-sized.
To partially solved such problems, an optical deflector of an air-magnetic bearing type is proposed in Japanese Patent Application No. 62-92048 having the same applicant as this application. In this optical deflector, a repulsive magnetic thrust bearing is disposed between an upper portion of a rotary body, an inner face of an upper wall of a casing and an upper portion of a fixed shaft.
This optical deflector of the air-magnetic bearing type solves a problem about the weak rigidity of a bearing of a simple repulsive type in the optical deflector using a rotary mirror shown in Japanese Patent Publication (KOKOKU) No. 53-6854.
However, in this optical deflector of the air-magnetic bearing type, a thrust bearing arranged in an upper end portion of the rotary shaft extends in the axial direction thereof and the number of parts is increased so that the optical deflector cannot be made compact and the cost thereof cannot be reduced.
Further, in a grinding process of the rotary shaft, it is necessary to attach and detach the rotary shaft from a working machine in the middle of the grinding work of the hollow rotary shaft. Therefore, it takes time to prepare and process the rotary shaft. Further, it is difficult to set right-hand and left-hand portions of a flange to be coaxial with respect to each other. Further, it takes time to perform a blank processing for forming the flange and many portions of a material of the rotary shaft become cut powder after the blanking processing so that there are many unnecessary portions in the material of the rotary shaft.
In addition to this, a polygon mirror is attached to an intermediate portion of the hollow rotary shaft so that an upper space of the rotary shaft is useless and the optical deflector cannot be made compact.